In the fabrication of modern semiconductor devices, the ever increasing device density and decreasing device dimensions demand more stringent requirements in the packaging or interconnecting techniques in such high density devices. Conventionally, a flip-chip attachment method has been used in packaging of semiconductor chips. In the flip-chip attachment method, instead of attaching a semiconductor die to a lead frame in a package, an array of solder bumps is formed on the surface of the die. The formation of the solder bumps may be carried out in an evaporation method by using a composite material of tin and lead through a mask for producing a desired pattern of solder bumps. The technique of electrodeposition has been more recently developed to produce solder bumps in flip-chip packaging process.
Other techniques that are capable of solder-bumping a variety of substrates to form solder balls have also been proposed. The techniques generally work well in bumping semiconductor substrates that contain solder structures over a minimal size. For instance, one of such widely used techniques is a solder paste screening method which has been used to cover the entire area of an eight inch wafer. However, with recent trend in the miniaturization of device dimensions and the necessary reduction in bump-to-bump spacing (or pitch), the use of the solder paste screening technique has become more difficult.
Other techniques for forming solder bumps such as the controlled collapse chip connection (C4) technique and the thin film electrodeposition technique have also been used in recent years in the semiconductor fabrication industry. The C4 technique is generally limited by the resolution achievable by a molybdenum mask which is necessary for the process. Fine-pitched solder bumps are therefore difficult to be fabricated by the C4 technique. Similarly, the thin film electrodeposition technique which also requires a ball limiting metallurgy layer to be deposited and defined by an etching process which has the same limitations as the C4 technique. For instance, a conventional thin film electrodeposition process for depositing solder bumps is shown in FIGS. 1A.about.1F.
A conventional semiconductor structure 10 is shown in FIG. 1A. The semiconductor structure 10 is built on a silicon substrate 12 with active devices built therein. A bond pad 14 is formed on a top surface 16 of the substrate 12 for making electrical connections to the outside circuits. The bond pad 14 is normally formed of a conductive metal such as aluminum. The bond pad 14 is passivated by a final passivation layer 20 with a window 22 opened by a photolithography process to allow electrical connection to be made to the bond pad 14. The passivation layer 20 may be formed of any one of various insulating materials such as oxide, nitride or organic materials. The passivation layer 20 is applied on top of the semiconductor device 10 to provide both planarization and physical protection of the circuits formed on the device 10.
Onto the top surface 24 of the passivation layer 20 and the exposed top surface 18 of the bond pad 14, is then deposited an under bump metallurgy layer 26. This is shown in FIG. 1B. The under bump metallurgy (UBM) layer 26 normally consists of an adhesion/diffusion barrier layer 30 and a wetting layer 28. The adhesion/diffusion barrier layer 30 may be formed of Ti, TiN or other metal such as Cr. The wetting layer 28 is normally formed of a Cu layer or a Ni layer. The UBM layer 26 improves bonding between a solder ball to be formed and the top surface 18 of the bond pad 14.
In the next step of the process, as shown in FIG. 1C, a photoresist layer 34 is deposited on top of the UBM layer 26 and then patterned to define a window opening 38 for the solder ball to be subsequently formed. In the following electrodeposition process, a solder ball 40 is electrodeposited into the window opening 38 forming a structure protruded from the top surface 42 of the photoresist layer 34. The use of the photoresist layer 34 must be carefully controlled such that its thickness is in the range between about 30 .mu.m and about 40 .mu.m, preferably at a thickness of about 35 .mu.m. The reason for the tight control on the thickness of the photoresist layer 34 is that, for achieving a fine-pitched solder bump formation, a photoresist layer of a reasonably small thickness must be used such that a high imaging resolution can be achieved. It is known that, during a photolithography process, the thicker the photoresist layer, the poorer is the imaging process. To maintain a reasonable accuracy in the imaging process on the photoresist layer 34, a reasonably thin photoresist layer 34 must be used which results in a mushroom configuration of the solder bump 40 deposited therein. The mushroom configuration of the solder bump 40 contributes greatly to the inability of a conventional process in producing fine-pitched solder bumps.
Referring now to FIG. 1E, wherein the conventional semiconductor structure 10 is shown with the photoresist layer 34 removed in a wet stripping process. The mushroom-shaped solder bump 40 remains while the under bump metallurgy layer 26 is also intact. In the next step of the process, as shown in FIG. 1F, the UBM layer 26 is etched away by using the solder bump 40 as a mask in an wet etching process. The solder bump 40 is then heated in a reflow process to form solder ball 42. The reflow process is conducted at a temperature that is at least the reflow temperature of the solder material.
In recent years, chip scale packages (CSP) have been developed as a new low cost packaging technique for high volume production of IC chips. One of such chip scale packaging techniques has been developed by the Tessera Company for making a so-called micro-BGA package. The micro-BGA package can be utilized in an environment where several of the packages are arranged in close proximity on a circuit board or a substrate much like the arrangement of individual tiles. Major benefits achieved by a micro-BGA package are the combined advantages of a flip chip assembly and a surface mount package. The chip scale packages can be formed in a physical size comparable to that of an IC chip even though, unlike a conventional IC chip such as a flip chip, the chip scale package does not require a special bonding process for forming solder balls. Furthermore, a chip scale package may provide larger number of input/output terminals than that possible from a conventional quad flat package, even though a typical quad flat package is better protected mechanically from the environment.
In a typical micro-BGA package, a flexible interposer layer (which may contain circuit) is used to interconnect bond pads on an IC chip to an array of solder bump connections located on a flexible circuit. The flexible circuit, normally of a thickness of approximately 25 .mu.m, is formed of a polymeric material such as polyimide which is laminated to a silicon elastomer layer of approximately 150 .mu.m thick. The silicon elastomeric layer provides flexibility and compliance in all three directions for relief of stresses and thermal expansion mismatches. To further reduce the fabrication cost of IC devices, it is desirable that if a whole wafer can be passivated to seal the IC dies on the wafer, and then be severed into individual IC dies from the wafer such that not only the benefits of a chip scale package can be realized, the packaging cost for the IC dies may further be reduced.
The conventional flip-chip bonding process requires multiple preparation steps for IC chips, i.e. the formation of aluminum bond pads on the chip, the under-bump-metallurgy process on the bond pads and the deposition of solder required in the bumping process. The substrate that the IC chip is bonded to requires a flux coating in order to ensure an acceptable bond strength is formed between the solder bumps and the conductive elements on the substrate surface. The flip chip bonding process further requires a reflow process for the bumps, a flux cleaning process to eliminate excess flux material from the surface of the bump, a drying process after the cleaning process, an underfill process for dispensing an underfill material, and an underfill curing process to minimize thermal stresses in the underfill and in the joint formed.
The conventional method for depositing solder bumps described above therefore presents a number of processing difficulties. For instance, one of the difficulties is the large volume of solder used to form a mushroom-shaped bump which impedes the process of making fine-pitched bumps. Another difficulty is the requirement of a flux coating step to ensure adhesion between a bump pad and a solder bump. The requirement of an additional UBM coating process further complicates the flip chip bonding method and increases its costs. It is therefore desirable to implement a flip chip bonding process that does not require separate processing steps for forming the solder bumps and for bonding the bumps to a substrate.
It is therefore an object of the present invention to provide a method for simultaneous bumping/bonding an IC chip to a substrate that does not have the drawbacks or shortcomings of the conventional bump forming and bonding method.
It is another object of the present invention to provide a method for simultaneous bumping/bonding an IC chip to a substrate utilizing edge-type conductive pads formed on the IC chip for bonding to conductive elements formed on a semiconductor substrate.
It is a further object of the present invention to provide a method for simultaneous bumping/bonding an IC chip to a substrate by first forming conductive pads on a wafer surface and then dicing the wafer to form edge-type conductive pads on each IC chip diced from the wafer.
It is another further object of the present invention to provide a method for simultaneous bumping/bonding an IC chip to a semiconductor substrate by first aligning edge-type conductive pads on an IC chip to conductive elements on a semiconductor substrate and then applying a solder therein between.
It is still another object of the present invention to provide a method for simultaneous bumping/bonding an IC chip to a semiconductor substrate by first forming conductive pads on a wafer surface and then dicing the wafer through a center line of the conductive pads.
It is yet another object of the present invention to provide a method for simultaneous bumping/bonding an IC chip to a semiconductor substrate by first forming a plurality of conductive pads on a wafer surface and then dicing the wafer into a plurality of IC chips each having at least one edge-type conductive pad formed on at least one edge.
It is still another further object of the present invention to provide a semiconductor package formed of an IC chip bonded to a substrate wherein the IC chip has a conductive pad exposed on a vertical edge of the chip bonded to a conductive element on a semiconductor substrate by a volume of solder.
It is yet another further object of the present invention to provide a semiconductor package formed of an IC chip bonded to a substrate wherein the IC chip has an edge-type conductive pad formed on a vertical edge of the chip by layers of under-bump-metallurgy materials such as a diffusion barrier layer and a wetting layer.